17997
2007, 111, 17997-18000 Published on Web 11/13/2007
Raman Spectroscopy of Nickel Sulfide Ni3S2 Zhe Cheng, Harry Abernathy, and Meilin Liu* School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst DriVe NW, Atlanta, Georgia 30332 ReceiVed: August 31, 2007; In Final Form: October 25, 2007
Raman spectra of nickel sulfides reported in previous studies are inconsistent due possibly to the difficulty in obtaining samples of the required purity in composition and phase. In this study, Raman spectra for both the low-temperature (heazlewoodite)- and the high-temperature (cubic)-phase Ni3S2, collected under both ex situ and in situ conditions, are correlated with factor group analysis and X-ray diffraction. The study is important to the investigation of sulfur-nickel interactions in catalyst systems for processing of sulfur-containing chemicals and in solid oxide fuel cells (SOFCs) that run on sulfur-containing fuels; it also provides a standard reference for research in areas such as the corrosion of nickel-based alloys in a sulfur-containing atmosphere.
Introduction In the study of gas/electrode interactions for solid oxide fuel cells (SOFCs), Raman spectroscopy is an effective tool in the detection of species formed on the electrode surface, especially under in situ conditions (i.e., at high temperature in a certain gas atmosphere).1-3 For nickel, the catalyst material for SOFC anodes, however, the problem of detecting and identifying surface sulfide species can be complicated because nickel sulfides have many different structures with different stoichiometries. Further, not all of the Raman spectra for the reported sulfides have been determined unambiguously. For example, Ni3S2, which commonly exists as the mineral heazlewoodite and is a typical corrosion product for nickel, has a rhombohedral structure with a space group of R32 (see JCPDS card No. 441418). The reported Raman spectra for the heazlewoodite-phase Ni3S2 conflict with each other.4-7 In addition, the Raman spectrum for the high-temperature cubic-phase Ni3S2 has never been reported. During the study of the sulfur-anode interactions in solid oxide fuel cells, we measured the Raman spectra for both the low-temperature (heazlewoodite)- and the high-temperaturephase Ni3S2 using in situ Raman spectroscopy. The results shown were correlated with factor group analysis, confirmed with X-ray diffraction (XRD), and compared with previous studies. These results provide a standard reference for the Raman spectra of Ni3S2, important for studying the interactions between sulfur and nickel (or nickel-based alloys) in a sulfur-containing atmosphere. Experimental Procedures The samples used in this study were dense composite pellets containing 40 vol % nickel and 60 vol % yttria-stabilized zirconia (YSZ). A nickel-YSZ composite was used because the study was carried out in the process of investigating the * To whom correspondence should be addressed. E-mail: meilin.liu@ mse.gatech.edu. Tel.: +1-404-894-6114.
10.1021/jp0770209 CCC: $37.00
interactions of sulfur with a typical Ni-YSZ SOFC anode. The individual nickel grain sizes were on the order of ∼20-60 µm. Since the Raman laser sample spot was focused to an intense spot of ∼1 µm diameter, this grain size allowed for the Raman measurement of the sulfide phase formed on the nickel surface without any interference from the neighboring YSZ phase. Raman spectra were collected using a Renishaw RM 1000 Raman microscope. The objective had a 50× magnification. The laser wavelength was usually 514 nm (Ar ion laser, Melles Griot), and the power of the laser beam at the focal point was ∼1 mW. For the in situ Raman experiment, a Ni-YSZ composite pellet, with a polished surface, was mounted onto the sample stage of a customized Raman sample chamber (modified HVC-DRP-1, Harrick Scientific) using a gold paste (C5755A, Heraeus). The Raman chamber was then heated in flowing hydrogen to the measuring temperature. Ni3S2 was formed in situ by exposing the Ni-YSZ composite to a flowing H2S/H2 fuel mixture with p(H2S)/p(H2) ) 10-4 (100 ppm) at ∼440 °C (the symbol “∼” is used since the error for temperature measurement was (5 °C). The Raman spectra, as well as optical images showing surface morphology, were recorded before and after 100 ppm H2S was introduced into the Raman chamber. To characterize Ni3S2 at temperatures greater than ∼440 °C using in situ Raman, a compact of commercial Ni3S2 (Aldrich #343226, which was actually a mixture of Ni3S2 and Ni7S6, as determined by XRD) was placed on a YSZ pellet placed on the sample cup. The sample was heated in a flowing fuel mixture of 50 ppm H2S/50% H2/50% N2 (by volume) to an elevated temperature up to ∼580 °C. (The phase transition between the low-temperature heazlewoodite-phase Ni3S2 and the hightemperature cubic-phase Ni3S2 happens at 565 ( 5oC.) X-ray diffraction (XRD) was carried out using a PW1800 X-ray diffractometer (Philips Analytical) at room temperature in air using Cu KR radiation. Results and Discussion Figure 1 shows the XRD pattern of the Ni-YSZ composite sample after being exposed to a flowing gas mixture of 100 © 2007 American Chemical Society
17998 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Letters
Figure 1. XRD pattern of the Ni-YSZ composite after exposure to 100 ppm H2S/H2 at ∼440 °C for 12 h and quickly cooled to room temperature.
Figure 3. Optical microscopy images of the Ni-YSZ composite before (a) and after (b) the formation of Ni3S2 on the Ni surface when it was exposed to a fuel of 100 ppm H2S/H2 at ∼440 °C for 12 h. The micron bar represents 20 µm.
Figure 2. Room-temperature Raman spectrum of Ni3S2 formed on a Ni surface after exposure to 100 ppm H2S/H2 at ∼440 °C for 12 h.
ppm H2S/H2 at ∼440 °C for 12 h and then quickly cooled to room temperature in a gas mixture of 4% H2/96% Ar at a rate of ∼70 °C/min. In addition to Ni and YSZ, which are the major components of the composite, Ni3S2 (heazlewoodite phase) and trace amounts of NiO were detected. The thickness of the Ni3S2 layer was estimated to be ∼1-5 µm from the roughness of the nickel surface. Figure 2 shows the corresponding roomtemperature Raman spectrum taken from the Ni regions, whose surface was covered by Ni3S2 and trace amounts of NiO. The spectrum has a total of six peaks at 188, 202, 224, 304, 325, and 350 cm-1. The Raman spectrum remained the same when the laser wavelength was changed from 514 to 488 nm. This spectrum is believed to correspond to heazlewoodite-phase Ni3S2 for several reasons. First, factor group analysis based on the correlation method given by Fateley et al.8 shows that the irreducible representation Γ of the phonon modes for the heazlewoodite-phase Ni3S2 (space group R32 (155) or D73) is R,IR 3S2,crystal ΓNi ) 2AR1 + 2AIR vib 2 + 4E
(1)
of which there are a total of six Raman-active bands, two A1 and four E. Second, the cubic NiO phase identified by XRD ) has a space group of Fm3m (225) or O5h, for which ΓNiO,crystal vib FIR 1u, which is not Raman active. In fact, previous studies revealed no sharp features in the 150-400 cm-1 region of the
Figure 4. Raman spectra taken from the Ni region before and after the formation of Ni3S2 during the in situ experiment at ∼440 °C in a fuel gas of 100 ppm H2S/H2.
Raman spectrum for NiO powders.9,10 The third piece of evidence can be seen in the in situ Raman data, which traced the formation of Ni3S2 on the Ni-YSZ composite surface at elevated temperatures in H2S-containing fuels. According to the phase diagram constructed by Rosenqvist and the thermal chemical data in the literature,11,12 Ni3S2 forms at ∼440 °C or below in gas mixtures with p(H2S)/p(H2)) 10-4. On the basis of those reported results, we carried out our in situ Raman experiment at ∼440 °C in a fuel mixture of 100 ppm H2S/H2. (We did not use a lower temperature in the range of ∼300400 °C because (i) the reaction kinetics would be slower and (ii) other nickel sulfides such as Ni7S6 or NiS would start to form in the same fuel at lower temperatures.) Figure 3 shows the change in the optical microscopy image of the Ni surface before and after Ni3S2 formation, while Figure 4 shows the corresponding change in the Raman spectrum taken from the same Ni region. The relative position and intensity of those peaks match those obtained at room temperature very well, except that every peak shifted to a lower wavenumber. For
Letters example, the peak at 350 cm-1 shifted to 341 cm-1 as the sample temperature increased from 20 to ∼440 °C, which was expected from the thermal expansion of the lattice.13 The peak at ∼325 cm-1 in Figure 2 is not visible probably because it is buried by the two adjacent peaks. The sharp peaks at 358, 589, and 816 cm-1 in those spectra are due to the rotation of physically adsorbed hydrogen.14 In addition to the experimental evidence provided above, the Raman spectrum for Ni3S2 has been confirmed by ab initio calculations, which provided assignment of the Raman bands.13 All of these results lead to the conclusion that the Raman spectrum shown in Figure 2 indeed corresponds to the heazlewoodite-phase Ni3S2. The spectrum obtained here for the heazlewoodite phase is different from most previous results in the literature.4-7 Ozkan et al. reported that their self-made Ni3S2 sample did not show any Raman bands.4 Zhu et al. claimed that Ni3S2 has one characteristic peak at 115 cm-1.5 Bishop reported that Ni3S2 (high-purity heazlewoodite, 97.98%, Aldrich Chemical Co.) has a total of 10 Raman peaks.6 Earlier, our own group, we now believe, incorrectly attributed Raman spectra with peaks at 208, 288, and 334 cm-1 to Ni3S2.15,16 The only match in the literature is the spectrum obtained using a 532 nm laser in the RRUFF database of Raman spectra, X-ray diffraction and chemistry data for minerals by the University of Arizona.7 It is not clear, however, why the researchers constructing the database obtained a quite different spectrum when using a 785 nm laser. Nevertheless, the huge discrepancy for the Raman spectrum of heazlewoodite-phase Ni3S2 in the literature illustrates the difficulty in obtaining high-purity Ni3S2 with the right phase by conventional methods. The nickel-sulfur system is very complicated, with several compounds existing.11,17-20 It is possible that the surface of the samples used by some other researchers did not have the exact stoichiometry and/or structure. Further, the surface of the sulfide sample may have been oxidized to NiO. In fact, as stated earlier, even the commercial Ni3S2 purchased from Aldrich by the authors turned out to be a mixture of Ni3S2 and Ni7S6. The Raman spectra obtained from some regions of the Aldrich “Ni3S2” are the same as that shown in Figure 2, but from some other regions, the spectra either contain a wide band in the range of 280 to 380 cm-1 (probably corresponding to Ni7S6) or resemble the spectrum of NiO. Such problems are solved by using an in situ setup that enables Raman measurement of the sample in a controlled environment after hours of exposure, ensuring the formation of the right phase desired from the phase diagram.11 According to the literature, Ni3S2 will go through a phase change from the low-temperature rhombohedral structure (heazlewoodite) to the high-temperature cubic structure at 565 ( 5°C.11,17-20 The Raman spectrum for the high-temperature cubicphase Ni3S2 has never been reported. In this study, the Raman spectroscopy for Ni3S2 at elevated temperatures was also investigated. The Raman spectra obtained near the phase transition temperature for Ni3S2 are shown in Figure 5. At temperatures below ∼563 °C, the Raman spectra were similar to those at room temperature (Figures 2 and 4), except most peaks shifted to a lower wavenumber. Once the temperature increased to ∼567 °C, the Raman peaks at 186, 205, 294, and 334 cm-1 disappeared suddenly; they reappeared when the temperature was reduced to ∼556 °C. According to JCPDS card No. 27-0341, the high-temperature form of Ni3S2 has a space group of F4h3m(216) or T2d. That JCPDS card assigns the number of molecules per unit cell (Z) to 2. Since the number of lattice points per unit cell for a facecentered cubic structure is 4, this would make the number of
J. Phys. Chem. C, Vol. 111, No. 49, 2007 17999
Figure 5. Raman spectra for Ni3S2 near the temperature of transition from the low-temperature rhombohedral (heazlewoodite) phase to the high-temperature cubic phase. The symbol “+” indicates the spectrum was taken during the heating process, while the symbol “-” indicates the spectrum was taken during the cooling process. The extra peak at ∼120 cm-1 is due to stray light, while the peak at 589 cm-1 is due to adsorbed hydrogen.14
molecules per lattice point equal to 0.5, which is hard to justify. By contrast, it has been proposed that the structure of the hightemperature cubic-phase Ni3S2 has a defective antifluorite structure.18-20 In the authors’ opinion, the later explanation is more reasonable. The real chemical formula for the hightemperature Ni3S2 would thus be Ni4-δS218 or, more precisely, Ni2-δS.19,20 This change makes the total number of molecules per unit cell equal to 4. The antifluorite structure has a space group of Fm3m(225) or O5h, for which ΓFluorite,crystal ) FIR vib 1u + R F2g. Of these modes, only F2g is a Raman-active mode,8 which matches the observation of a broad peak at ∼320 cm-1 as in Figure 5. However, the broad nature of this peak makes the use of the Raman spectrum as a fingerprint for Ni3S2 at temperatures greater than 565 ( 5 °C less attractive; the presence of Ni3S2 is hard to judge without cooling below the transition temperature to observe the more distinct heazlewoodite signal, but cooling the sample may not be desirable for some in situ measurements. Conclusions The Raman spectra for the low-temperature (heazlewoodite) phase and the high-temperature (cubic) phase of Ni3S2 have been characterized using ex situ and in situ Raman spectroscopy at different temperatures in controlled atmospheres. The results were analyzed using factor group analysis and compared with the data in the literature. It was demonstrated that with in situ Raman, the difficulty of obtaining pure samples with the right phase in the complex Ni-S system could be overcome and the spectra for nickel sulfides could be obtained with much higher credibility. Acknowledgment. This work was supported by the U.S. DOE-NETL SECA Core Technology Program (Grant No. DEFC26-04NT42219) and DOE Office of Basic Energy Sciences, Catalysis Science Program (Grant No. DE-FG02-06ER15837).
18000 J. Phys. Chem. C, Vol. 111, No. 49, 2007 References and Notes (1) Pomfret, M. B.; Owrutsky, J. C.; Walker, R. A. J. Phys. Chem. B 2006, 110, 17305. (2) Pomfret, M. B.; Owrutsky, J. C.; Walker, R. A. Anal. Chem. 2007, 79, 2367. (3) Cheng, Z.; Liu, M. Solid State Ionics 2007, 178, 925. (4) Ozkan, U. S.; Zhang, L.; Ni, S.; Moctezuma, E. Energy Fuels 1994, 8, 830. (5) Zhu, X.; Li, J.; Wadsworth, M.; Woods, R. Electrochemical Kinetics of the Anodic Dissolution of Nickel Matte and Synthetic Ni3S2. In Hydrometallurgy and Refining of Nickel and Cobalt, Proceedings of the 27th Annual Hydrometallurgy Meeting of CIM, Sudbury, Ontario, Canada, Aug. 17-20, 1997; Cooper, W. C., Mihaylov, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Quebec, 1997; p 153. (6) Bishop, D. W.; Thomas, A. S.; Ray, A. S. Mater. Res. Bull. 2000, 35, 1123. (7) RRUFF Database of Raman spectra, X-ray Diffraction and Chemistry Data for Minerals. http://rruff.info/index.php. The University of Arizona. (8) Fateley, W. G.; Dollish, F. R.; McDevitt, N. T.; Bentley, F. F. Infrared and Raman Selection Rules for Molecular and Lattice Vibrations: The Correlation Methods; John Wiley & Sons: New York, 1972.
Letters (9) Dietz, R. E.; Parisot, G. I.; Meixner, A. E. Phys. ReV. B 1971, 4, 2302. (10) Melendres, C. A.; Xu, S. J. Electrochem. Soc. 1984, 130, 2239. (11) Rosenqvist, T. J. Iron Steel Inst. 1954, 176, 37. (12) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.; American Institute of Physics: New York, 1986. (13) Wang, J. H.; Cheng, Z.; Bre´das, J.-L.; Liu, M. J. Chem. Phys. In press. (14) Colpa, J. P.; Ketelaar, J. A. Mol. Phys. 1958, 1, 14. (15) Dong, J.; Zha, S.; Liu, M. Study of Sulfur-Nickel Interactions Using Raman Spectroscopy. In Proceedings of the 9th International Symposium on Solid Oxide Fuel Cells; Singhal, S. C., Mizusaki, J., Eds.; The Electrochemical Society: Pennington, NJ, 2005; Vol. 2005-07, p 1284. (16) Dong, J.; Cheng, Z.; Zha, S.; Liu, M. J. Power Sources 2006, 156, 461. (17) Kullerud, G.; Yund, R. A. J. Petrol. 1962, 3, 126. (18) Line, G.; Huber, M. Compt. Rend. 1963, 256, 3118. (19) Stølen, S.; Grønvold, F.; Westrum, E. F.; Kolonin, G. R. J. Chem. Thermodyn. 1991, 23, 77. (20) Fjellvåg, H.; Andersen, A. Acta Chem. Scand. 1994, 48, 290.
